2,698 research outputs found

    Dyon condensation in topological Mott insulators

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    We consider quantum phase transitions out of topological Mott insulators in which the ground state of the fractionalized excitations (fermionic spinons) is topologically non-trivial. The spinons in topological Mott insulators are coupled to an emergent compact U(1) gauge field with a so-called "axion" term. We study the confinement transitions from the topological Mott insulator to broken symmetry phases, which may occur via the condensation of dyons. Dyons carry both "electric" and "magnetic" charges, and arise naturally in this system because the monopoles of the emergent U(1) gauge theory acquires gauge charge due to the axion term. It is shown that the dyon condensate, in general, induces simultaneous current and bond orders. To demonstrate this, we study the confined phase of the topological Mott insulator on the cubic lattice. When the magnetic transition is driven by dyon condensation, we identify the bond order as valence bond solid order and the current order as scalar spin chirality order. Hence, the confined phase of the topological Mott insulator is an exotic phase where the scalar spin chirality and the valence bond order coexist and appear via a single transition. We discuss implications of our results for generic models of topological Mott insulators.Comment: 14 pages, accepted to the New Journal of Physic

    Demonstrating the model nature of the high-temperature superconductor HgBa2_2CuO4+Δ_{4+\Delta}

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    The compound HgBa2_2CuO4+Δ_{4+\Delta} (Hg1201) exhibits a simple tetragonal crystal structure and the highest superconducting transition temperature (Tc_c) among all single Cu-O layer cuprates, with Tc_c = 97 K (onset) at optimal doping. Due to a lack of sizable single crystals, experimental work on this very attractive system has been significantly limited. Thanks to a recent breakthrough in crystal growth, such crystals have now become available. Here, we demonstrate that it is possible to identify suitable heat treatment conditions to systematically and uniformly tune the hole concentration of Hg1201 crystals over a wide range, from very underdoped (Tc_c = 47 K, hole concentration p ~ 0.08) to overdoped (Tc_c = 64 K, p ~ 0.22). We then present quantitative magnetic susceptibility and DC charge transport results that reveal the very high-quality nature of the studied crystals. Using XPS on cleaved samples, we furthermore demonstrate that it is possible to obtain large surfaces of good quality. These characterization measurements demonstrate that Hg1201 should be viewed as a model high-temperature superconductor, and they provide the foundation for extensive future experimental work.Comment: 15 pages, 6 Figure

    Influence of oxygen vacancy on the electronic structure of HfO2_2 film

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    We investigated the unoccupied part of the electronic structure of the oxygen-deficient hafnium oxide (HfO1.8_{\sim1.8}) using soft x-ray absorption spectroscopy at O KK and Hf N3N_3 edges. Band-tail states beneath the unoccupied Hf 5dd band are observed in the O KK-edge spectra; combined with ultraviolet photoemission spectrum, this indicates the non-negligible occupation of Hf 5dd state. However, Hf N3N_3-edge magnetic circular dichroism spectrum reveals the absence of a long-range ferromagnetic spin order in the oxide. Thus the small amount of dd electron gained by the vacancy formation does not show inter-site correlation, contrary to a recent report [M. Venkatesan {\it et al.}, Nature {\bf 430}, 630 (2004)].Comment: 5 pages, 4 figures, submitted to Phys. Rev.

    Quantum dot-doped porous silicon metal–semiconductor metal photodetector

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    In this paper, we report on the enhancement of spectral photoresponsivity of porous silicon metal–semiconductor metal (PS-MSM) photodetector embedded with colloidal quantum dots (QDs) inside the pore layer. The detection efficiency of QDs/PS hybrid-MSM photodetector was enhanced by five times larger than that of the undoped PS-MSM photodetector. The bandgap alignment between PS (approximately 1.77 eV) and QDs (approximately 1.91 eV) facilitates the photoinduced electron transfer from QDs to PS whereby enhancing the photoresponsivity. We also showed that the photoresponsitivity of QD/PS hybrid-MSM photodetector depends on the number of layer coatings of QDs and the pore sizes of PS.Published versio

    Isotropic three-dimensional gap in the iron-arsenide superconductor LiFeAs from directional heat transport measurements

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    The thermal conductivity k of the iron-arsenide superconductor LiFeAs (Tc ~ 18K) was measured in single crystals at temperatures down to T~50mK and in magnetic fields up to H=17T, very close to the upper critical field Hc2~18T. For both directions of the heat current, parallel and perpendicular to the tetragonal c-axis, a negligible residual linear term k/T is found as T ->0, revealing that there are no zero-energy quasiparticles in the superconducting state. The increase in k with magnetic field is the same for both current directions and it follows closely the dependence expected for an isotropic superconducting gap. There is no evidence of multi-band character, whereby the gap would be different on different Fermi-surface sheets. These findings show that the superconducting gap in LiFeAs is isotropic in 3D, without nodes or deep minima anywhere on the Fermi surface. Comparison with other iron-pnictide superconductors suggests that a nodeless isotropic gap is a common feature at optimal doping (maximal Tc).Comment: 4 pages, 3 figure

    Embolic Infarction Associated with Cardiac Amyloidosis

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    Embolic cerebral infarction due to cardiac amyloidosis is rare. We report two patients with amyloidosis who developed cerebral infarcts. These embolic infarcts were probably related to cardiac involvement of amyloidosis, which was based on results of myocardial biopsy (Patient 1), and kidney biopsy and characteristic echocardiographic features including granular sparkling, restrictive cardiomyopathy and the presence of mural thrombus (Patient 2). Diffuse amyloid infiltration of the heart may have lead to impairment of myocardial function and subsequent mural thrombosis. Cardiomyopathy due to cardiac amyloidosis should be recognized as one of the causes of cardioembolic infarction
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